Electronic sensors for determining the direction of an external magnetic field are well known in the art in a variety of contexts. One particularly important use of such sensors is to determine the orientation of the sensor with respect to the magnetic field of the earth. When such a sensor is employed in this way it is often called an electronic compass. Electronic compasses have been developed using one or more magnetic fluxgates to sense the external magnetic field. The basic fluxgate compass is an electromagnetic sensor that employs two or more small coils of wire wrapped around a core of non-linear magnetic material to directly sense the horizontal component of the earth's magnetic field.
Electronic compasses have numerous advantages over conventional mechanical compasses utilizing a piece of magnetized metal to indicate direction. One such advantage is that an electronic compass may be made much smaller in size than a magnetized metal mechanical compass. Another advantage is that electronic compasses are not affected by acceleration or deceleration of a vehicle in which the compass is carried. An additional advantage is that an electronic compass provides an electrical output allowing a simple interface with other electronic circuitry such as an electronic navigation system or an autopilot.
The electrical output may, for example, be digitized and visually displayed. The digitized directional reading may be electronically compensated to correct for directional errors due to surrounding ferrous metal and nearby magnetic emissions sources such as iron-bearing ores present in natural geological formations. If multiple fluxgate detectors are used in a fluxgate array, the digitized output may also be corrected for magnetic variation resulting from the earth's magnetic field dipping downward toward the poles, or for magnetic deviation, that is, the difference between true north and magnetic north, which is a function of longitude, latitude, elevation and date.
To avoid directional inaccuracies created by the vertical component of the earth's magnetic field, a fluxgate array must be kept as flat as possible by mounting it on gimbals or using a fluid suspension system. Nevertheless, inertial errors are inevitable when the compass is turning sharply or rolled from side to side. Such roll and heel-induced errors often plague fluxgate compasses installed on watercraft and vehicles designed to operate over rough terrain. Another disadvantage of fluxgate detector arrays is that such systems depend on the individual fluxgate sensors having substantially identical operating characteristics if the output of the sensors is to be used directly without substantial additional signal processing to correct for differences in such operating characteristics. Typical fluxgate detector fabrication processes do not produce such uniform sensors, however.
It is desirable to provide a low cost, easy to make and use, and enhanced sensitivity magnetic field sensor. It would also be highly desirable to fabricate a compass that does not exhibit the deficiencies associated with known flux-gate compasses. It would thus be highly desirable to fabricate a MEMS compass on a single wafer.